London: For the first time, researchers have witnessed a fatal flight among primates in which a female orangutan recruited a male as a hired gun to help her corner and attack another female.
Before this observation, lethal fights between females had never been observed among orangutans -- in other primates such fights occur mainly between males, said lead study author Anna Marzec from University of Zurich in Switzerland.
The fatal incident was reported in Springer's journal Behavioural Ecology and Sociobiology.
The research team had been following a population of Bornean orangutans in the swamp forests of Indonesia's Mawas Reserve since 2003 and already collected over 26,000 hours of information on the adult females alone when they observed the fatal attack in July 2014.
During this period, only six female-female attacks had been observed, none of which had caused visible injuries. Other long-term studies of orangutans similarly have never reported such violent female attacks.
The case involved Kondor, a young female who had lost her infant just weeks before, and Sidony, a much older resident female who did not interact much with neighbouring apes.
The two females had a history of aggressive interaction.
In the week before the lethal attack, Kondor was seen with a male called Ekko. The two of them encountered Sidony and her dependent son.
After Ekko sexually inspected Sidony, he returned to Kondor to mate with her. Kondor interrupted these sexual activities when Sidony started to move away, and attacked her.
Ekko joined the fight, which lasted 33 minutes. They continuously attacked as a coordinated team.
While one attacked, the other blocked the victim's escape route. Kondor instigated two further shorter attacks. Ekko, who had long canines typical of a male, inflicted the most serious injuries and effectively prevented Sidony's escape.
The dynamics changed when another male, Guapo, arrived and chased Ekko away. Guapo then mated with Sidony.
Kondor continued to harass and bite her. Although Guapo successfully protected her from further damaging attacks, Sidony died two weeks later.
"This is quite unexpected, as in wild orangutans males and females have never been reported to form coalitions before," Marzec said.